The M&P Sport simply can't be beat for value in an assembled rifle.
That said, I think you'd do well to put one together for her, using a midlength upper. Middy rifles just plain recoil softer. Not that a 5.56 carbine is harsh, but the impulse is noticeably sharper in a rifle of equal weight.
My suggestion:
DSA stripped lower: $75 + Shipping & Transfer
DSA or Del-Ton Mid Length upper: Some variation here depending on exactly what you want, but lowest cost will be $384 for a DSA ZM4 Middy upper ($250), DSA Bolt Carrier Group ($120) and DSA standard charging handle ($14).
Stock kit: Standard 6-position M4 stock & extension kits are $50 & up for decent stuff. Or you can get just a receiver extension kit for $30 from DSA and put whatever stock you want on it.
PSA Lower Parts Kit: The kit with Magpul MOE grip and trigger guard is only $50 right now (or it was last week when I ordered 2).
Magazine: No brainer. Magpul P-mag 30, about $15.
So, you could build her a basic mid length rifle for as little as $575 plus shipping and transfer. Shipping should be about $30. FFL transfer varies but $20-$40 is normal. So $625-$645 all said and done; You won't beat that for an assembled middy rifle. (A Del-Ton rifle kit would come in about $30 lower, but they've been perpetually out of stock for months now)
If you can assemble a cabinet from Ikea, you can assemble an AR. It's fun, you gain a better understanding of what's going on in there, and you can say you built it for her. She may even want to participate (mine did).
Links:
https://www.dsarms.com/
http://www.del-ton.com/
http://Palmettostatearmory.com